Application of reduced dark current photodetector with a thermoelectric cooler

ABSTRACT

A IDCA system combining thermo-electric cooler (TEC) and an internal nBn photo-detector having a photo absorbing layer comprising an n-doped semiconductor exhibiting a valence band energy level; and a contact layer comprising a doped semiconductor. A barrier layer is disposed between the photo absorbing layer and the contact layer, the barrier layer exhibiting a valence band energy level substantially equal to the valence band energy level of the doped semiconductor of the photo absorbing layer; the barrier layer exhibiting a thickness and a conductance band gap sufficient to prevent tunneling of majority carriers from the photo absorbing layer to the contact area and block the flow of thermalized majority carriers from the photo absorbing layer to the contact area. Alternatively, a p-doped semiconductor is utilized, and conductance band energy levels of the barrier and photo absorbing layers are equalized.

This application is related to, and hereby incorporates by reference thefollowing Patents and Patent Applications in their entirety: U.S. Pat.Nos. 7,687,871; 8,003,434; U.S. patent application Ser. Nos. 13/033,211;13/622,981; 13/964,883; and 13/167,992.

FIELD OF THE INVENTION

The invention relates generally to the field of semiconductor basedphoto-detectors and in particular to a photo-detector exhibiting abarrier region between an active semiconductor region and a contactsemiconductor region.

BACKGROUND OF THE INVENTION

Photo-detectors are used in a wide variety of applications includingimaging. A specific type of photo-detector sensitive to the infra-redwavelengths of light is also known as an infra-red detector. Infra-redcovers a broad range of wavelengths, and many materials are onlysensitive to a certain range of wavelengths. As a result, the infra-redband is further divided into sub-bands such as near infra-red definedconventionally as 0.75-1.4 μm; short wavelength infra-red definedconventionally as 1.3-3 μm; mid wavelength infra-red definedconventionally as 3-8 μm; and far infra-red defined conventionally as15-1,000 μm. Infra-red in the range of 5 μm to 8 μm is not welltransmitted in the atmosphere and thus for many infra-red detectionapplications mid-wavelength infra-red is referred to as 3-5 μm.

Infra-red detectors are used in a wide variety of applications, and inparticular in the military field where they are used as thermaldetectors in night vision equipment, air borne systems, naval systemsand missile systems. Highly accurate thermal detectors have beenproduced using InSb and HgCdTe p-n junction diodes, however thesethermal detectors require cooling to cryogenic temperatures of around 77K which is costly. Examples of these existing technologies are presentedin FIG. 5A to FIG. 5F. The cryogenic temperatures primarily are used toreduce the dark current generated in the p-n junction diode by amongother effects Shockley Reed Hall (SRH) generation.

There are three main contributions to the dark current, denoted asI_(dark), of photodiodes based on narrow band gap semiconductors. Thefluctuations of the dark current components are a major factor in thenoise that limits the device performance. These components are:

-   -   a) a generation current associated with the Shockley-Reed-Hall        (SRH) process in the depletion region, I_(srh);    -   b) a diffusion current associated with auger or radiative        processes in the extrinsic area, I_(diff); and    -   c) a surface current associated with the surface states in the        junction, I_(surf). The surface current depends primarily on the        passivation process done for the device. Thus, I_(dark) can be        expressed as:        I _(dark) =I _(srh) +I _(diff) +I _(surf)  Equation 1

The SRH generation process is very efficient in the depletion region ofphotodiodes where the mid-gap traps are highly activated. It is the mainsource of the dark current in photodiodes operable for mid-wavelengthinfrared at temperatures below 200K. The current associated with thissource is:

$\begin{matrix}{J_{SRH} \approx {q\;\frac{n_{i}}{\tau_{SRH}}W_{dep}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

-   -   where n_(i) is the intrinsic concentration of the semiconductor,        W_(dep) is the depletion width (typically in the range of 1 μm),        and τ_(SRH) is the SRH lifetime of minority carriers in the        extrinsic area. The SRH lifetime of minority carriers in the        extrinsic area depends on the quality of the material, i.e. the        trap concentration, and is typically in the range of ˜1 μsec in        low doped material (˜10¹⁶ cm⁻³). The dependence of SRH current        on n_(i) produces an activation energy of

${{E_{g}/2}\left( {\left. n_{i} \right.\sim{\exp\left( {- \frac{\frac{E_{g}}{2}}{kT}} \right)}} \right)},$

-   -    because the source of this generation process is through        mid-gap traps. A secondary source of dark current in photodiodes        is thermal generation in the neutral regions and diffusion to        the other side of the junction. This thermal generation current        depends on the auger or radiative process in this area, and is        expressed as:

$\begin{matrix}{{J_{diff} \approx {{qp}_{n} \times \frac{1}{\tau_{diff}} \times L}} = {q \times \frac{n_{1}^{2}}{N_{d}} \times \frac{1}{\tau_{diff}} \times L}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

-   -   where τ_(diff) is the lifetime, and in an n-type material        exhibiting a doping concentration, denoted N_(d), of ˜1−2·10¹⁶        cm⁻³ is in the range of ˜0.5 μsec, depending only slightly on        temperature. L is the width of the neutral region of the device        or the diffusion length of minority carriers (the smaller of the        two) and p_(n) is the hole concentration in the active n type        semiconductor in equilibrium and it equal to n_(i) ²/N_(d). The        activation energy of the diffusion current is E_(g),

$\left( {\left. n_{i}^{2} \right.\sim{\exp\left( {- \frac{E_{g}}{kT}} \right)}} \right)$

-   -    as the process involves band to band excitation.

Additionally, p-n junction diodes, and particularly those produced forthermal imaging require a passivation layer in the metallurgic junctionbetween the p and n layers. Unfortunately this is often difficult toachieve and significantly adds to the cost of production.

There is thus a long felt need for a thermal imaging device that uses aphoto-detector with reduced dark noise. Preferably the photo-detectorwould be sensitive to the mid wavelength infra-red band and not requireexpensive passivation in production. Further preferably thephoto-detector would be operable at significantly higher temperaturesthan 77K. Further preferably the thermal imaging device would be able tooperate for longer periods, be lighter and require less power, whencompared to the existing technology in the art.

SUMMARY OF THE INVENTION

Accordingly, it is a principal object of the present disclosure toovercome the disadvantages of the existing technological deficiencies ofphoto-detectors and their application within thermal imaging devices,with particular reference to mid and long wavelength infra-reddetectors. This is facilitated in the present invention by aphoto-detector sensitive to a target waveband comprising a photoabsorbing layer, preferably exhibiting a thickness on the order of theoptical absorption length. In an exemplary embodiment the photoabsorbing layer is deposited to a thickness of between one and two timesthe optical absorption length. A contact layer is further provided, anda barrier layer is interposed between the photo absorbing layer and thecontact layer. The barrier layer exhibits a thickness sufficient toprevent tunneling of majority carriers from the photo absorbing layer tothe contact layer, and a band gap barrier sufficient to block the flowof thermalized majority carriers from the photo absorbing layer to thecontact layer. The barrier layer does not significantly block minoritycarriers.

An infra-red detector in accordance with the principle of the inventioncan be produced using either an n-doped photo absorbing layer or ap-doped photo absorbing layer, in which the barrier layer is designed tohave no offset for minority carriers and a band gap barrier for majoritycarriers. Current in the detector is thus almost exclusively by minoritycarriers. In particular, for an n-doped photo absorbing layer thejunction between the barrier layer and the absorbing layer is such thatthere is substantially zero valence band offset, i.e. the band gapdifference appears almost exclusively in the conduction band offset. Fora p-doped photo absorbing layer the junction between the barrier layerand the absorbing layer is such that there is substantially zeroconduction band offset, i.e. the band gap difference appears almostexclusively in the valence band offset.

Advantageously the photo-detector of the subject invention does notexhibit a depletion layer, and thus the dark current is significantlyreduced. Furthermore, in an exemplary embodiment passivation is notrequired as the barrier layer further functions to achieve passivation.

An exemplary photo-detector of the present disclosure comprises: a photoabsorbing layer comprising an n-doped semiconductor exhibiting a valenceband energy level and a conducting band energy level; a barrier layer, afirst side of the barrier layer adjacent a first side of the photoabsorbing layer, the barrier layer exhibiting a valence band energylevel substantially equal to the valence band energy level of the photoabsorbing layer and a conduction band energy level exhibiting asignificant band gap in relation to the conduction band of the photoabsorbing layer; and a contact area comprising a doped semiconductor,the contact area being adjacent a second side of the barrier layeropposing the first side, the barrier layer exhibiting a thickness, thethickness and the band gap being sufficient to prevent tunneling ofmajority carriers from the photo absorbing layer to the contact area andblock the flow of thermalized majority carriers from the photo absorbinglayer to the contact area.

In one embodiment of the photo detector the barrier layer comprises anundoped semiconductor. In another embodiment the contact area isn-doped. In a further embodiment, the contact area exhibits a valenceband energy level substantially equal to the valence band energy levelof the n-doped semiconductor of the photo absorbing layer.

In one embodiment of the photo detector the contact area is p-doped. Inone further embodiment the contact area exhibits a valence band energylevel greater than the valence band energy level of the n-dopedsemiconductor of the photo absorbing layer. In another furtherembodiment the barrier layer comprises an undoped semiconductor.

In one embodiment of the photo detector the photo absorbing layer isoperable to generate minority carriers in the presence of light energyexhibiting a wavelength of 3-5 microns. In another embodiment thephoto-detector further comprises a substrate exhibiting a first sideadjacent a second side of the photo absorbing layer, the second side ofthe photo absorbing layer opposing the first side of the photo absorbinglayer, the substrate exhibiting a second side in contact with a metallayer. Preferably, the photo-detector further comprises an additionalmetal layer in contact with the contact area.

In one embodiment of the photo detector the barrier layer comprises oneof AlSb, AlAsSb, GaAlAsSb, AlPSb, AlGaPSb and HgZnTe. In a furtherembodiment the photo absorbing layer is constituted of one of n-dopedInAs, n-doped InAsSb, n-doped InGaAs, n-doped Type II super latticeInAs/InGaSb and n-doped HgCdTe. In a yet further embodiment of the photodetector the contact area is constituted of one of InAs, InGaAs, InAsSb,Type II super lattice InAs/InGaSb, HgCdTe and GaSb. In a yet furtherembodiment the contact area and the photo absorbing layer exhibitsubstantially identical compositions.

In one embodiment of the photo detector the photo absorbing layer andthe contact area arc constituted of n-doped HgCdTe and the barrier layeris constituted of HgZnTe, and in another embodiment the photo absorbinglayer and the contact layer are constituted of n-doped type II superlattice InAs/InGaSb and the barrier layer is constituted of AlGaAsSb.

In another embodiment of the photo detector the photo absorbing layer isconstituted of n-doped InAsSb, the barrier layer is constituted ofAlGaAsSb and the contact layer is constituted of p-doped GaSb. In oneembodiment the photo absorbing layer exhibits a thickness on the orderof the optical absorption length.

Another embodiment of a photo-detector comprises: a photo absorbinglayer comprising a p-doped semiconductor exhibiting a conduction bandenergy level and a valence band energy level; a barrier layer, a firstside of the barrier layer adjacent a first side of the photo absorbinglayer, the barrier layer exhibiting a conduction band energy levelsubstantially equal to the conduction band energy level of the photoabsorbing layer and a valence band energy level exhibiting a significantband gap in relation to the valence band of the photo absorbing layer;and a contact area comprising a doped semiconductor, the contact areaadjacent a second side of the barrier layer opposing the first side, thebarrier layer exhibiting a thickness, the thickness and the band gapbeing sufficient to prevent tunneling of majority carriers from thephoto absorbing layer to the contact area and to block the flow ofthermalized majority carriers from the photo absorbing layer to thecontact area.

In one embodiment of a photo-detector the barrier layer comprises anundoped semiconductor. In another embodiment the contact area isp-doped. In one further embodiment of a photo-detector the contact areaexhibits a conduction band energy level substantially equal to theconduction band energy level of the p-doped semiconductor of the photoabsorbing layer.

In one embodiment of a photo-detector the contact area is n-doped. Inone further embodiment the contact area exhibits a conduction bandenergy level less than the conduction band energy level of the p-dopedsemiconductor of the photo absorbing layer. In another furtherembodiment the barrier layer comprises an undoped semiconductor.

In one embodiment of a photo-detector the photo absorbing layer isoperable to generate minority carriers in the presence of light energyexhibiting a wavelength of 3-5 microns. In another embodiment thephoto-detector further comprises a substrate exhibiting a first sideadjacent a second side of the photo absorbing layer, the second side ofthe photo absorbing layer opposing the first side of the photo absorbinglayer, the substrate exhibiting a second side in contact with a metallayer. In a further embodiment the photo-detector further comprises ametal layer in contact with the contact area.

In one embodiment of a photo-detector the barrier layer comprises one ofAlSb, AlAsSb, GaAlAsSb, AlPSb, AlGaPSb, InAlAs, InAlAsSb, and HgZnTe. Inone further embodiment the photo absorbing layer is constituted of oneof p-doped InAs, p-doped InAsSb, p-doped InGaAs, p-doped Type II superlattice InAs/InGaSb and p-doped HgCdTe. In one yet further embodimentthe contact area is constituted of one of InAs, InGaAs, InAsSb, Type IIsuper lattice InAs/InGaSb, HgCdTe and GaSb. In one yet furtherembodiment the contact area and the photo absorbing layer exhibitsubstantially identical compositions.

An exemplary method of producing a photo-detector, comprises: providinga substrate; depositing on the substrate a photo absorbing layercomprising a doped semiconductor exhibiting an energy level associatedwith non-conducting majority carriers; depositing on the deposited photoabsorbing layer a barrier layer exhibiting a thickness, an energy levelassociated with minority carriers of the photo absorbing layersubstantially equal to the energy level of the photo absorbing layer anda band gap associated with majority carriers of the photo absorbinglayer; and depositing on the deposited barrier layer a contact layercomprising a doped semiconductor, the thickness and the band gap of thebarrier layer being sufficient to prevent tunneling of majority carriersfrom the photo absorbing layer to the contact layer and to block theflow of thermalized majority carriers from the photo absorbing layer tothe contact layer.

In one embodiment the method further comprises selectively etching thedeposited contact layer to define a plurality of contact areas. Inanother embodiment at least one of depositing the photo absorbing layer,depositing the barrier layer and depositing the contact layer is donevia one of molecular beam epitaxy, metal organic chemical vapordeposition, metal organic phase epitaxy and liquid phase epitaxy.

It is noted that while the photodetector is preferably manufactured on asubstrate as described, certain embodiments may remove the substrate ora portion thereof in the final photodetector, and thus in an embodimentof the invention there is provided a photo detector.

Additional features and advantages of the invention will become apparentfrom the following drawings and description.

SHORT DESCRIPTION OF DRAWINGS

For a better understanding of the invention and to show how the same maybe carried into effect, reference will now be made, purely by way ofexample, to the accompanying drawings in which like numerals designatecorresponding elements or sections throughout.

With specific reference now to the drawings in detail, it is stressedthat the particulars shown are by way of example and for purposes ofillustrative discussion of the preferred embodiments of the presentinvention only, and are presented in the cause of providing what isbelieved to be the most useful and readily understood description of theprinciples and conceptual aspects of the invention. In this regard, noattempt is made to show structural details of the invention in moredetail than is necessary for a fundamental understanding of theinvention, the description taken with the drawings making apparent tothose skilled in the art how the several forms of the invention may beembodied in practice. In the accompanying drawings:

FIG. 1A illustrates a high level schematic view of the layers of asingle photo-detector according to an embodiment of the principle of theinvention;

FIG. 1B illustrates a side view of a multi-pixel photo-detectoraccording to an embodiment of the principle of the invention;

FIG. 1C illustrates a top level view of the multi-pixel photo-detectorof FIG. 1B according to a principle of the invention;

FIG. 2A illustrates the energy band levels of an embodiment of thestructure of FIG. 1 according to the principle of the invention in whichthe photo absorbing layer is n-doped and the contact layer is n-doped;

FIG. 2B illustrates the energy band levels of an embodiment of thestructure of FIG. 1 according to the principle of the invention in whichthe photo absorbing layer is p-doped and the contact layer is p-doped;

FIG. 3A illustrates the energy band levels of an embodiment of thestructure of FIG. 1 according to the principle of the invention in whichthe photo absorbing layer is n-doped and the contact layer is p-doped;

FIG. 3B illustrates the energy band levels of an embodiment of thestructure of FIG. 1 according to the principle of the invention in whichthe photo absorbing layer is p-doped and the contact layer is n-doped;and

FIG. 4 illustrates a high level flow chart of the process of manufactureof the multi pixel photo-detector of FIG. 1B-1C.

FIG. 5a , 5B, 5C, 5D, 5E and 5F illustrate examples of existingapplications of the prior-art photo-detector technology.

FIG. 6 illustrates the existing external components of an exemplarythermal imaging device with an integrated dewar cooler assembly,hereafter IDCA.

FIG. 7 illustrates the internal components of an exemplary applicationof the disclosed subject matter.

FIG. 8 illustrates an exemplary linear micro cooler (Ricor's K527 split)that is utilized in conjunction with the disclosed subject matter.

FIG. 9 illustrates resultant improvement in cooling power requirements.

FIG. 10A illustrates a mother board joined to FPA.

FIG. 10B illustrates a FPA and motherboard located within IDCAarrangement.

FIG. 11 illustrates a schematic application of the disclosed subjectmatter. FIG. 11A depicts schematically a single stage TEC, and FIG. 11Bdepicts a multi-stage TEC. FIG. 11C depicts schematically a FPA 1114disposed on a TEC 1145.

FIG. 12 illustrates an operational flow chart of an exemplary opticalimaging device with an IDCA comprising the disclosed FDA.

DETAILED DESCRIPTION

The present embodiments enable a photo-detector sensitive to a targetwaveband comprising a photo absorbing layer, preferably exhibiting athickness on the order of an optical absorption length of the targetwaveband. In an exemplary embodiment the photo absorbing layer isdeposited to a thickness of between one and two times the opticalabsorption length. A contact layer is further provided, and a barrierlayer is interposed between the photo absorbing layer and the contactlayer. The barrier layer exhibits a thickness sufficient to preventtunneling of majority carriers from the photo absorbing layer to thecontact layer, and a band gap barrier sufficient to block the flow ofthermalized majority carriers from the photo absorbing layer to thecontact layer. The barrier layer does not significantly block minoritycarriers.

An infra-red detector in accordance with the principle of the inventioncan be produced using either an n-doped photo absorbing layer or ap-doped photo absorbing layer, in which the barrier layer is designed tohave substantially no offset for minority carriers and a band gapbarrier for majority carriers. Current in the detector is thus almostexclusively by minority carriers. In particular, for an n-doped photoabsorbing layer the junction between the barrier layer and the absorbinglayer is such that there is substantially zero valence band offset, i.e.the band gap difference appears almost exclusively in the conductionband offset. For a p-doped photo absorbing layer the junction betweenthe barrier layer and the absorbing layer is such that there issubstantially zero conduction band offset, i.e. the band gap differenceappears almost exclusively in the valence band offset.

Advantageously the photo-detector of the subject invention does notexhibit a depletion layer, and thus the dark current is significantlyreduced. Furthermore, in an exemplary embodiment passivation is notrequired as the barrier layer further functions to achieve passivation.

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not limited in its applicationto the details of construction and the arrangement of the components setforth in the following description or illustrated in the drawings. Theinvention is applicable to other embodiments or of being practiced orcarried out in various ways. Also, it is to be understood that thephraseology and terminology employed herein is for the purpose ofdescription and should not be regarded as limiting.

FIG. 1A illustrates a high level schematic view of the layers of aphoto-detector 10 according to an embodiment of the principle of theinvention comprising a substrate 20, a photo absorbing layer 30, abarrier layer 40, a contact layer 50, a metal layer 60 and a metal layer65. Substrate 20 is provided as a base for deposition and has depositedon one face metal layer 60 for connection to electronic circuitry. In anexemplary embodiment metal layer 60 is constituted of gold. Photoabsorbing layer 30 is deposited on the second face of substrate 20opposing the first face. Photo absorbing layer 30 comprises a dopedsemiconductor responsive to photons of the object wavelength, andpreferably is deposited to a thickness on the order of an opticalabsorption length. In one embodiment photo absorbing layer 30 isdeposited to a thickness of between one and two times the opticalabsorption length. In an exemplary embodiment photo absorbing layer 30comprises one of n-doped InAs; n-doped InAsSb; n-doped InGaAs; n-dopedtype II super lattice of the type InAs/InGaSb; and n-doped HgCdTe. In analternative embodiment absorbing layer 30 comprises one of p-doped InAs;p-doped InAsSb; p-doped InGaAs; p-doped type II super lattice of thetype InAs/InGaSb; and p-doped HgCdTe.

Barrier layer 40 is deposited directly on photo absorbing layer 30without requiring passivation. Barrier layer 40 is deposited to athickness sufficient to substantially prevent tunneling of majoritycarriers from photo absorbing layer 30 to contact layer 50, and in anexemplary embodiment is deposited to a thickness of 50-100 nm. Barrierlayer 40 comprises a material selected to exhibit a high band gapbarrier for majority carriers from photo absorbing layer 30 andsubstantially no band gap barrier for minority carriers. Barrier layer40 is thus sufficient to block both the flow of thermalized majoritycarriers and the tunneling of majority carriers from photo absorbinglayer 30 to contact layer 50. Thus, for an n-type photo absorbing layer30, the band gap difference appears in the conduction band, whereassubstantially no band gap offset appears in the valence band. In oneembodiment barrier layer 40 comprises one of AlSb, AlAsSb, GaAlAsSb,AlPSb, AlGaPSb and HgZnTe. In an exemplary embodiment photo absorbinglayer 30 comprises n-doped InAs and barrier layer 40 is comprised ofAlAs_(x)Sb_(1−x) with x˜0.15, and thus there is ˜0 valence band offset.

Contact layer 50 is deposited on barrier layer 40. Contact layer 50functions to absorb the minority carriers diffused from the absorbinglayer 30 and is essentially a contact layer. In an exemplary embodimentcontact layer 50 is deposited to a thickness of 20-50 nm and isconstituted of one of InAs; InAsSb; InGaAs; type II super lattice of thetype InAs/InGaSb; HgCdTe and GaSb. Contact layer 50 may be n-doped orp-doped without exceeding the scope of the invention. Advantageously,contact layer 50 may be constituted of the same material as photoabsorbing layer 30. Contact layer 50 is etched, preferably byphotolithography, to define the detector area. Advantageously etching ofbarrier layer 40 or absorbing layer 30 is not required. Metal layer 65is deposited on contact layer 50, and in an exemplary embodiment isconstituted of gold. Metal layers 60, 65 enable the connection of anappropriate bias, and a connection to detect a flow of current fromphoto absorbing layer 30 to contact layer 50.

FIG. 1B illustrates a side view of a multi-pixel photo-detector 100according to an embodiment of the principle of the invention comprisingsubstrate 20, photo absorbing layer 30, barrier layer 40, a first andsecond contact area 110, a metal layer 6 and a metal layer 65. Substrate20 is provided as a base for deposition and has deposited on one facemetal layer 60 for connection to electronic circuitry. In an exemplaryembodiment metal layer 60 is constituted of gold. Photo absorbing layer30 is deposited on the second face of substrate 20 opposing the firstface. Photo absorbing layer 30 comprises a doped semiconductorresponsive to photons of the object wavelength, and preferably isdeposited to a thickness on the order of an optical absorption length.In one embodiment photo absorbing layer 30 is deposited to between oneand two times the optical absorption length. In an exemplary embodimentphoto absorbing layer 30 comprises one of n-doped InAs; n-doped InAsSb;n-doped InGaAs; n-doped type II super lattice of the type InAs/InGaSb;and n-doped HgCdTe. In an alternative embodiment absorbing layer 30comprises one of p-doped InAs; p-doped InAsSb; p-doped InGaAs; p-dopedtype II super lattice of the type InAs/InGaSb; and p-doped HgCdTe.

The substrate is generally transparent to wavelengths of interest.However as certain substrates may block certain portions of thespectrum, in some embodiments the substrate, or portions thereof, may beremoved to allow all the spectrum of interest to be detected by thephotodetector array.

Barrier layer 40 is deposited directly on photo absorbing layer 30without requiring passivation. Barrier layer 40 is deposited to athickness sufficient to substantially prevent tunneling of majoritycarriers from photo absorbing layer 30 to first and second contact area110, and in an exemplary embodiment is deposited to a thickness of50-100 nm. Barrier layer 40 comprises a material selected to exhibit ahigh band gap barrier for majority carriers from photo absorbing layer30 and substantially no band gap barrier for minority carriers. Barrierlayer 40 is thus sufficient to block both the flow of thermalizedmajority carriers and the tunneling of majority carriers from photoabsorbing layer 30 to first and second contact area 110. Thus, for ann-type photo absorbing layer 30, the band gap difference appears in theconduction band, whereas substantially no band gap offset appears in thevalence band. In one embodiment barrier layer 40 comprises one of AlSb,AlAsSb, GaAlAsSb, AlPSb, AlGaPSb and HgZnTe. In an exemplary embodimentphoto absorbing layer 30 comprises n-doped InAs and barrier layer 40 iscomprised of AlAs_(x)Sb_(1−x) with x˜0.15, and thus there is ˜0 valenceband offset.

Contact layer 50 as described above in relation to FIG. 1A is depositedon barrier layer 40. Contact layer 50, which as will be describedfurther is etched to define first and second contact area 110, functionsto absorb the minority carriers diffused from the absorbing layer 30 andis essentially a contact layer. In an exemplary embodiment contact layer50 is deposited to a thickness of 20-50 nm and is constituted of one ofInAs; InAsSb; InGaAs; type II super lattice of the type InAs/InGaSb;HgCdTe and GaSb. Contact layer 50 may be n-doped or p-doped withoutexceeding the scope of the invention. Advantageously, contact layer 50may be constituted of the same material as photo absorbing layer 30.Contact layer 50 is etched, preferably by photolithography, to definefirst and second contact area 110. Advantageously etching of barrierlayer 40 or absorbing layer 30 is not required. In an exemplaryembodiment a selective etchant is used which does not etch barrier layer40. Metal layer 65 is deposited on each of first and second contact area110, and in an exemplary embodiment is constituted of gold. Thus, asingle photo absorbing layer and barrier layer is utilized, with eachunetched portion of contact layer 50 defining a pixel or individualdetector.

The above has been described in an embodiment in which two pixels, ordetectors are defined, however this is not meant to be limiting in anyway. A large array of photo-detectors produced as above is specificallyincluded in the invention.

FIG. 1C illustrates a top level view of multi-pixel photo-detector 100of FIG. 1B according to a principle of the invention showing barrierlayer 40, first and second contact area 110 and metal layer 65 definedon each of first and second contact area 110.

FIG. 2A illustrates the energy band levels of an embodiment of thestructure of FIG. 1 according to the principle of the invention in whichthe photo absorbing layer is n-doped and the contact layer is n-doped,in which the x-axis indicates position along the structure of FIG. 1 andthe y-axis indicates energy levels in an arbitrary illustrative manner.Three energy band levels are depicted: E_(v), the valence band energyband level; E_(f), the Fermi energy band level; and E_(c) the conductingband energy level. Area 100 represents the energy band levels withinphoto absorbing layer 30, area 110 represents the energy band levelswithin barrier layer 40 and area 120 represent the energy band levelswithin contact layer 50.

The valence band energy level is substantially constant throughout areas100, 110 and 120, and thus minority carriers are not obstructed fromflowing from photo absorbing area 100 to contact area 120. It is to benoted that due to the energy levels the minority carriers are capturedin contact area 120. Barrier layer 40, represented by area 110, is thickenough so that there is negligible tunneling of majority carriersthrough it. In an exemplary embodiment barrier layer 40 is deposited toa thickness of 50-100 nm, and the band gap barrier of area 110 is highenough so that there is negligible thermal excitation of majoritycarriers over it. Area 120 shows energy band levels on a par with thatof area 100 however this is not meant to be limiting in any way. In oneembodiment E_(f) in contact layer area 120 is slightly higher than theirvalues in photo absorbing area 100 with the increase being attributed toan increased doping concentration. It is to be noted that no depletionlayer is present and therefore there is no SRH current. Photocurrent isa result of optically generated minority carriers which diffuse fromphoto absorbing area 100 to contact area 120.

FIG. 2B illustrates the energy band levels of an embodiment of thestructure of FIG. 1 according to the principle of the invention in whichthe photo absorbing layer is p-doped and the contact layer is p-doped;in which the x-axis indicates position along the structure of FIG. 1 andthe y-axis indicates energy levels in an arbitrary illustrative manner.Three energy band levels are depicted: E_(v), the valence band energylevel; E_(f), the Fermi energy band level; and E_(c) the conducting bandenergy level. Area 150 represents the energy band levels within photoabsorbing layer 30, area 160 represents the energy band levels withinbarrier layer 40 and area 170 represent the energy band levels withincontact layer 50.

The conduction band energy level is substantially constant throughoutareas 150, 160 and 170, and thus minority carriers are not obstructedfrom flowing from photo absorbing area 150 to contact area 170. It is tobe noted that due to the energy levels the minority carriers arecaptured in contact area 170. Barrier layer 40, represented by area 160,is thick enough so that there is negligible tunneling of majoritycarriers through it. In an exemplary embodiment barrier layer 40 isdeposited to a thickness of 50-100 nm, and the band gap barrier of area160 is high enough so that there is negligible thermal excitation ofmajority carriers over it. Area 170 shows energy band levels on a parwith that of area 150 however this is not meant to be limiting in anyway. In one embodiment E_(f) in contact layer area 170 is slightlyhigher than their values in photo absorbing area 150 with the increasebeing attributed to an increased doping concentration. It is to be notedthat no depletion layer is present and therefore there is no SRHcurrent. Photocurrent is a result of optically generated minoritycarriers which diffuse from photo absorbing area 150 to contact area170.

FIG. 3A illustrates the energy band levels of an embodiment of thestructure of FIG. 1 according to the principle of the invention in whichthe photo absorbing layer is n-doped and the contact layer is p-doped;in which the x-axis indicates position along the structure of FIG. 1 andthe y-axis indicates energy levels in an arbitrary illustrative manner.Three energy band levels are depicted: E_(v), the valence band energylevel; E_(f), the Fermi energy band level; and E_(c) the conducting bandenergy level. Area 200 represents the energy band levels within photoabsorbing layer 30, area 210 represents the energy band levels withinbarrier layer 40 and area 220 represent the energy band levels withincontact layer 50.

The valence band energy level is substantially constant throughout areas200 and 210 and is higher in area 220, and thus minority carriers arenot obstructed from flowing from photo absorbing area 200 to contactarea 220. It is to be noted that due to the energy levels the minoritycarriers are captured in contact area 220. Barrier layer 40, representedby area 210, is thick enough so that there is negligible tunneling ofmajority carriers through it. In an exemplary embodiment barrier layer40 is deposited to a thickness of 50-100 nm, and the band gap barrier ofarea 210 is high enough so that there is negligible thermal excitationof majority carriers over it. It is to be noted that no depletion layeris present and therefore there is no SRH current. Photocurrent is aresult of optically generated minority carriers which diffuse from photoabsorbing area 200 to contact area 220.

FIG. 3B illustrates the energy band levels of an embodiment of thestructure of FIG. 1 according to the principle of the invention in whichthe photo absorbing layer is p-doped and the contact layer is n-doped;in which the x-axis indicates position along the structure of FIG. 1 andthe y-axis indicates energy levels in an arbitrary illustrative manner.Three energy band levels are depicted: E_(v), the valence band energylevel; E_(f), the Fermi energy band level; and E_(c) the conducting bandenergy level. Area 250 represents the energy band levels within photoabsorbing layer 30, area 260 represents the energy band levels withinbarrier layer 40 and area 270 represent the energy band levels withincontact layer 50.

The conduction band energy level is substantially constant throughoutareas 250 and 260 and it is lower in area 270, and thus minoritycarriers are not obstructed from flowing from the photo absorbing area250 to contact area 270. It is to be noted that due to the energy levelsthe minority carriers are captured in contact area 270. Barrier layer40, represented by area 260, is thick enough so that there is negligibletunneling of majority carriers through it. In an exemplary embodimentbarrier layer 40 is deposited to a thickness of 50-100 nm, and the bandgap barrier of area 260 is high enough so that there is negligiblethermal excitation of majority carriers over it. It is to be noted thatno depletion layer is present and therefore there is no SRH current.Photocurrent is a result of optically generated minority carriers whichdiffuse from photo absorbing area 250 to contact area 270.

FIG. 4 illustrates a high level flow chart of the process of manufactureof the photo-detector of FIG. 1. In stage 1000 a substrate material isprovided as a support for deposition. In stage 1010, a photo absorbinglayer is deposited on the substrate. Preferably the photo absorbinglayer is deposited to a thickness on the order of the optical absorptionlength and in an exemplary embodiment is deposited to a thickness ofbetween one and two times the optical absorption length.

In stage 1020, a barrier material is selected such that the flow ofthermalized majority carriers from the photo absorbing layer depositedin stage 1010 would be negligible, and the flow of minority carriers isnot impeded. In stage 1030, the barrier material selected in stage 1020is deposited to a thickness sufficient to prevent tunneling of majoritycarriers through the barrier material. In an exemplary embodiment thethickness is between 50 and 100 nm. Preferably the barrier material isdeposited directly on the photo absorbing layer deposited in stage 1010.

In stage 1040, a contact layer is deposited, preferably directly on thebarrier material deposited in stage 1030. In stage 1050, the desiredcontact areas are defined. Preferably, the contact areas are defined byphotolithography and a selective etchant which stops on the top of thebarrier layer. Alternatively, the etchant may be controlled to stop oncethe uncovered portions of contact layer 50 are removed. Thus, the depthof the etch is equivalent to the thickness of the contact layer 50.Advantageously, in an exemplary embodiment no other layer is etched.

In stage 1060 a metal layer is deposited on the contact areas defined instage 1050 so as to enable electrical connection. Preferably the metallayer is deposited directly on the contact areas defined in stage 1050.In stage 1070, a metal layer is deposited on substrate 20 provided instage 1000 so as to enable electrical connection.

Deposition of the photo absorbing layer of stage 1010, the barrier layerof stage 1030 and the contact layer of stage 1040 may be accomplished byany means known to those skilled in the art including, withoutlimitation molecular beam epitaxy, metal organic chemical vapordeposition, metal organic phase epitaxy or liquid phase epitaxy.

Thus the present embodiment enable a photo-detector sensitive to atarget waveband comprising a photo absorbing layer, preferablyexhibiting a thickness on the order of the optical absorption length. Inan exemplary embodiment the photo absorbing layer is deposited to athickness of between one and two times the optical absorption length. Acontact layer is further provided, and a barrier layer is interposedbetween the photo absorbing layer and the contact layer. The barrierlayer exhibits a thickness sufficient to prevent tunneling of majoritycarriers from the photo absorbing layer to the contact layer, and a bandgap barrier sufficient to block the flow of thermalized majoritycarriers from the photo absorbing layer to the contact layer. Thebarrier layer does not block minority carriers.

An infra-red detector in accordance with the principle of the inventioncan be produced using either an n-doped photo absorbing layer or ap-doped photo absorbing layer, in which the barrier layer is designed tohave no offset for minority carriers and a band gap barrier for majoritycarriers. Current in the detector is thus almost exclusively by minoritycarriers. In particular, for an n-doped photo absorbing layer thejunction between the barrier layer and the absorbing layer is such thatthere is substantially zero valence band offset, i.e. the band gapdifference appears almost exclusively in the conduction band offset. Fora p-doped photo absorbing layer the junction between the barrier layerand the absorbing layer is such that there is substantially zeroconduction band offset, i.e. the band gap difference appears almostexclusively in the valence band offset.

Advantageously the photo-detector of the subject invention does notexhibit a depletion layer, and thus the dark current is significantlyreduced. Furthermore, in an exemplary embodiment passivation is notrequired as the barrier layer further functions to achieve passivation.

An exemplary application of the disclosed subject matter is theinclusion of an array of photo detectors within a focal plan array,hereafter FPA, which form an integral component of optical imagingdevices, including thermal imaging devices. Use of the disclosed subjectmatter within the FPA enables improved thermal imaging deviceperformance, including but not limited to, weight, duration ofoperation, power requirements, cost, pixel operability and durability.

FIGS. 5A, 5B, 5C and 5D present hand-held imaging systems, which utilizethe existing photo detector technology.

FIGS. 5E and 5F present aviation technology, which utilize the existingphoto detector technology, including the Lockheed Sniper Pod and theNorthrop Grumman EOTS pod.

FIG. 6 presents a further example of the existing technology, whereinlight enters the apparatus through the front lens optic 602, interactswith the apparatus internal electroptic componentary 604, where it isconverted from infra-red light to a electric signal, which istransmitted and presented on the apparatus' display 606.

FIG. 7 presents in greater detail the essential components of anexemplary integrated dewar cooler system, hereafter IDCS. In oneembodiment, light enters the system 700 through the front optic 704, thelight than is received by the FPA, which is located on the cold fingerof the dewar 702, and is maintained at a cryogenic temperature. In oneembodiment of the disclosure, the FPA operates at a temperature of 150K.In other embodiments, the FPA can operate across a temperature spectrumof between 77K and 150K. The IDCS further comprises a micro-cooler 706,which is responsible for the refiguration of the FPA.

FIG. 8 presents an exemplary split linear micro cooler system,comprising a cooler 902 connected via tubing 904 to a cold finger 906,and an external controller 908 connected via wiring 910 to cooler. Theexemplary cooler system presented is the Ricor k527 split linear microcooler. In other embodiment a variety of micro cooler systems can beused.

FIG. 9 illustrates the micro-cooler power consumption per heat loadacross a spectrum of cold finger temperatures.

FIG. 10A presents the FPA 1002 coupled to a motherboard 1004, whichforms a FPA arrangement that can connect to the cold finger.

FIG. 10B presents an IDCS in greater detail, wherein light enters thedewar 1114 via the optical window 1110, with a micro-cooler 1102providing refrigeration for the FPA.

FIG. 11 illustrates an exemplary schematic IDCA arrangement. The systemoperates by allowing light through the front element 1110 which is anoptical window located behind a fronting lens, not shown. Light entersthrough the optical window before interacting with the FPA 1114. The FPAarrangement comprises an array of photo-detectors, coupled to amotherboard. The FPA arrangement is contained within the cold finger ofa cryogenic vacuum sealed dewar 1116-1112, which is refrigerated by themicro cooler 1102. The dewar additional comprising: a getter unit 1104;a cold shield 1108 contained within the window envelope 1112, which arepositioned on the cold finger 1116. Light interacts with the FPAarrangement, which produces a photo-electronic signal in response. Thephoto-electronic signal is in turn transmitted from the FPA arrangementvia wiring to the feed-through pins 1106.

The IDCA arrangements described heretofore for utilize gas displacementbased cryogenics system, such as Stirling cycle systems, pulse tubesystems, and the like. Such systems are heavy, relatively complex andexpensive. However the low dark current of detectors according to thepresent invention allow utilization of thermo-electric coolers (TEChereinafter) utilizing the Peltier effect. TEC's are light relativelyinexpensive, and require no moving parts which increase system lifetime. Certain TEC devices are capable of reaching as low as 150 K andbelow, which allow practical uses of detectors according to the presentinvention in many applications such as the applications described hereinand extending to other devices as well, such as, by way of non-limitingexample, heart beat sensing cameras, firefighters heat sensing cameras,health care systems, and potentially even in cellular telephone devices,as well as other heat sensing applications in the military andcommercial applications. It is noted that such weight savings would behighly appreciated in the military field, making heat sensing deviceslight enough to be carried by the individual soldier, without reducingthe soldier's fighting ability. Weight saving is also of extremeimportance to the aerospace industry, and this aspect of the presentinvention extends to light sensing equipment in all those fields, whileutilizing photodetectors in accordance with the present inventioncoupled to a TEC.

FIG. 11A depicts schematically a single stage TEC, and FIG. 11B depictsa multi-stage TEC. FIG. 11C depicts schematically a FPA 1114 inaccordance with the present invention, disposed on a TEC 1145. A coldshield 1147 surrounds the FPA so as to prevent spurious radiation fromthe dewar side walls into the sensor. The TEC 1145, the FPA 1114 and thecold shield 1147 are disposed within a dewar 1149 having an opticalwindow. Notably, due to the relatively high operating temperature, thedewar does not have to be a vacuum dewar, and instead may be filled withinert gas. However a vacuum dewar is also explicitly contemplated.

FIG. 12 presents an flow chart diagram of an exemplary operationalprocess of the disclosed subject matter, comprising: light beinggenerated at source 1202; light entering the apparatus through the frontlens element 1204; light interacting with the FPA 1206; the FPAgenerating a electro message 1208; the electro message being transmittedfrom the FPA to the IDCA's feeder tubes 1210; the electro message beingreceived by the devices electronics 1214; wherein the device eitherdisplay the image 1216 or transmits the image to an external displaydevice 1218.

In one embodiment, the optical imaging device containing the IDCA has aninterchangeable front lens element. In other embodiment, the front lenselement may be fixed, may be fixed and variable, and other arrangementsas standard in the art.

In one embodiment of the disclosed subject matter, the IDCA with anarray of improved photo detector is accommodated within amateur,professional, or commercial optical devices. In other embodiments, theIDCA is located within military equipment. Aviation examples includeprecision targeting devices, or Electro Optic Targeting Systems (EOTS).

In yet another embodiment, the IDCA can comprise a plurality of FPA.

Examples of the above disclosure included but is not limited to,incorporation of the claimed IDCA within: Lockheed Sniper PodTechnology; Lockheed EOTS pods; AN/AAQ-37 F-35 Distributed ApertureSystem (DAS made by Northrop Grumman) and other similar technology;hand-held personal cameras; professional cameras; and security opticaldevices; another example is missile seeker. The disclosed IDCA apparatuscould also be incorporated into the existing technologies outlined inthe background of the invention.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention which are, for brevity, described in thecontext of a single embodiment, may also be provided separately or inany suitable sub-combination.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meanings as are commonly understood by one of ordinaryskill in the art to which this invention belongs. Although methodssimilar or equivalent to those described herein can be used in thepractice or testing of the present invention, suitable methods aredescribed herein.

All publications, patent applications, patents, and other referencesmentioned herein are incorporated by reference in their entirety. Incase of conflict, the patent specification, including definitions, willprevail. In addition, the materials, methods, and examples areillustrative only and not intended to be limiting.

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed hereinabove. Rather the scope of the present invention isdefined by the appended claims and includes both combinations andsub-combinations of the various features described hereinabove as wellas variations and modifications thereof, which would occur to personsskilled in the art upon reading the foregoing description.

What is claimed is:
 1. An integrated dewar cooler system comprising: alight permitting optical window; a sealed Dewar; a thermoelectricCooler; and a focal plane array, wherein said focal plane array is anarray of photo detectors with a reduced dark current, comprising: aphoto absorbing layer comprising a doped semiconductor exhibiting avalence band energy and a conducting band energy during operation of thephoto-detector; a barrier layer comprising an undoped semiconductor, thebarrier layer having a band energy gap and associated conduction andvalence band energies, a first side of said barrier layer adjacent afirst side of said photo absorbing layer; and a contact layer comprisinga doped semiconductor exhibiting a valence band energy and a conductingband energy during operation of the photo-detector, said contact layerbeing adjacent a second side of said barrier layer opposing said firstside; wherein the relationship between the photo absorbing layer andcontact layer valence and conduction band energies and the barrier layerconduction and valance band energies facilitates minority carriercurrent flow while inhibiting majority carrier current flow between thecontact and photo absorbing layers.
 2. An integrated dewar cooler asclaimed in claim 1, wherein the dewar is at least partially evacuated.3. An integrated dewar cooler as claimed in claim 1 wherein the dewar isat least partially filled with an inert gas.